1. Field of the Invention
This invention relates to a dynamic pressure gas bearing device for use in the rotational unit of a business machine, an image instrument, an information instrument, an optical instrument or the like.
2. Description of the Prior Art
Heretofore, a structure in which the opposite ends of a shaft on which a polygon mirror is mounted are supported by ball bearings has often been used in a bearing device for the rotational unit of a rotational polygon mirror light deflector used in an instrument such as a laser beam printer. However, in a spindle device using the conventional ball bearings, it is not easy to attain the required accuracy of rotation, an irregularity of rotation results because of vibration based on the error in the machining of the race for the ball bearings, the vibration caused by the passage of the balls, and the vibration caused by the retainer or the irregularity of rotation caused by the grease enclosed in the ball bearings being irregularly bitten by the balls in rotation are unavoidable.
In such a rotational polygon mirror light deflector, where the accuracy of rotation of the polygon mirror in highspeed rotation is poor and where there is irregularity of rotation of the polygon mirror, the characters printed become blurred and, therefore, very severe accuracy of dynamic rotation is required of the supporting bearings. However, with the recent tendency of printers toward a higher speed and a smaller size, the number of revolutions of the rotational polygon mirror light deflector has increased from several thousand rpm to several tens of thousand rpm and the apparatus itself is in the tendency toward a smaller size. It has therefore become more and more difficult to improve the accuracy of rotation and eliminate the irregularity of rotation. Also, with the tendency toward a higher speed, the life of the ball bearings has become shorter and problems in reliability have arisen.
Further, to prevent the polygon mirror from being stained, it is desired that a lubricant such as grease which may scatter or evaporate not be used for the support bearings. However, the deterioration of the performance of the polygon mirror by scattering or evaporation of grease is unavoidable with the ball bearings because the ball bearings are greaselubricated. Even if magnetic fluid seal is used, scattering or evaporation of the oil itself used in the magnetic fluid is unavoidable and the use of such seal cannot be an essential countermeasure. Also, in the case of the ball bearings, prepressure adjustment has been necessary and therefore assembly has not always been easy, and it has been difficult from the viewpoint of mass production to assemble the ball bearings so that there is no mounting error to maintain dynamic rotational accuracy.